Problem: In physics class, the girl to boy ratio is $10$ to $8$. If there are a total of $36$ students, how many girls are there?
A ratio of $10$ girls to $8$ boys means that a set of $18$ students will have $10$ girls and $8$ boys. A class of $36$ students has $2$ sets of $18$ students. Because we know that there are $10$ girls in each set of $18$ students, the class must have $2$ groups of $10$ girls each. There is a total of $20$ girls in physics class.